Apparatus for measuring the acuity of low-vision patients

ABSTRACT

An optical system for measuring the acuity of a low-vision patient has a variably magnified image of a reference character displayed on a rear vision screen. The image is effectively rotatable in order to provide the operator with a verification of the response of the patient.

trite States Patent Grolman Feb. 1,1972

[54] APPARATUS FOR MEASURING THE ACUITY OF LOW-VISION PATIENTS [72]Inventor: Bernard Grolman, Worcester, Mass.

[73] Assignee'. American Optical Corporation,

Southbridge, Mass.

[22] Filed: June 10, 1970 [21] Appl No.: 45,190

[52] U.S. Cl ..35l/30 [51] Int. Cl. ..A61b 3/02 [58] Field of Search..35 1/30-32; 350/188, 187, 3.5

[56] References Cited UNITED STATES PATENTS 3,490,832 1/1970 Mitsuishiet al ..351/30 3,341,275 9/1967 l-laugen ..351/3O 2,924,146 2/1960 Back..350/187 1,174,547 3/1916 Clason ..351/30 OTHER PUBLICATIONS Leith,Emmett et al., Holographic Imagery Through Diffusing Media, JOSA, Vol.56, No. 4, April 1966, p. 523.

Primary ExaminerDavid Schonberg Assistant ExaminerPaul A. SacherAttorney-William C. Nealon, Noble S. Williams, Robert J. Bird andBernard L. Sweeney [5 7] ABSTRACT An optical system for measuring theacuity of a low-vision patient has a variably magnified image of areference character displayed on a rear vision screen, The image iseffectively rotatable in order to provide the operator with averification of the response of the patient.

5 Claims, 2 Drawing Figures INVENTOR. BERNARD GROLMAN APPARATUS FORMEASURING THE ACUITY OF LOW- VISION PATIENTS BACKGROUND OF THE INVENTIONThis invention is related to ophthalmic testing instruments and is moreparticularly concerned with improved apparatus for measuring the acuityof low-vision patients.

The accurateascertainment of the acuity of low-vision pa.- tients isimportant for a number of reasons. Among these is the determination ofthe efficacy of optical appliances, such as eyeglasses, which may beprescribed-for the patientln other cases, the determination of theacuity of sucha patient is necessary for satisfying the requirements forthe condition of legal blindness. In each of the above cases, theaccuracy and repeatability of the test results are extremely important.

According to current practices, the standard means of acquiring such adetermination involves the use of a letter chart, generally of theSnellen type, which is mounted on a moveable stand. The chart is movedtoward and away from SUMMARY OF THE INVENTION Accordingly, it is anobject of the present invention to provide an optical system foraccurately measuring the acuity of a low-vision patient.

Another object of the invention is to provide such a system in which avariably magnified image of a reference character is presented on ascreen, the screen being fixed relative to the patient.

A further object of the invention is to provide such an apparatus inwhich a zoom lens system is utilized to provide the necessary variablemagnification and in which the patient or the clinician is able toremotely control the magnification of the image displayed on the screen.

A still further object of the invention is to provide such an apparatusin which means are provided for accomplishing a direct readout of apatients threshold level.

Briefly, the invention in its broadest aspect comprises apparatus formeasuring the acuity of a low-vision patient. The

apparatus includes a target having a reference character DESCRIPTION OFTHE DRAWING In the drawing,

FIG. I is a schematic, isometric view of apparatus according to theinvention, and I FIG. 2 is an enlarged showing of a preferred referencecharacter for use with the apparatus of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG.1, there is shown apparatus for measuring the acuity of a low-visionpatient. The apparatus 10 is generally comprised of a target 12, a rearvision screen 18, a means 16 for projecting a variably magnified imageof a reference character on the target 12 onto the rear vision screen18, and a means 17 for controlling the magnification of the image. Themeans 16 for projecting a variably magnified image of the character ontothe rear vision screen 18 includes, in this preferred embodiment, a zoomlens system 20, a projectionv lamp 22, and a projection lens 24. Theprojection system 16 provides an image of the character, on the targetMi an 12, on the rear ofthe screen 18. The patient, who is positioned infront of the screen 18, views the image presented thereon and informsthe operator when he is able to recognize the character presented.

The zoom lens system 20 is comprised of a central rotatable sleeve 26-,on which a cam groove 28 is formed, and a pair of annular end caps 30.The end caps are held fixed by a pair of brackets 32 and 34. The zoomlens system 20 includes an optical system such as is shown in US. Pat.No. 3,502,392 to Muller, which is also assigned to the assignee of thepresent application. The disclosure of this patent is incorporatedherein by reference to show an example of a feasible zoom optical systemfor use in the present invention. The particular mechanical and opticalcomponents form no part of the present invention.

Means 17 is provided for adjusting the magnification of the imagepresented on the screen 18. For example, a cam follower 36, whichengages the camming groove 28, is mounted on a lead screw 38 which is inturn journaled in a pair of hearing brackets 40; A pulley 42 is mountedon the end of the lead screw shaft and is connected by means of a belt44 to the output shaft 46 of a reversible electric motor 48. The motor48 is controllable by means of a rotary switch 50 which is connected tothe motor by means of conductors 52. The switch 50 may be located suchthat either the clinician or the patient may operate theelectromechanical means in order to vary the magnification of the imagepresented. This reversible rotation of the switch 50 is represented bythe arrow 54. However, any other means of controlling the magnificationis also included within the purview of the invention, e.g., manualrotation of the sleeve 26..

A preferred reference character for incorporation in the target 12 isshown by the illiterate E" 14 set forth in FIG. 2. A Landolt ring isalso seen as a preferred reference character to allow the clinician toperform ophthalmic examinations on illiterate patients, such aschildren, as well as other individuals. Also, it is more difficult for apatient to falsify the test results by prior knowledge of the charactersto be presented. The patient is presented with different sizes of thecharacter in a number of different rotational orientations. In addition,the test results are more valid, as many patients are able to recognizeletters by their general shape even though they cannot actually discernthe character presented. With the aforementioned preferred referencecharacters, the general shape is always the same, i.e., a square or acircle, and the patient must be able to see the character quite clearlyto ascertain the correct orientation.

The clinician may wish to verify the patients response. This isaccomplished by effectively rotating the reference character displayedon the target 12. The target 12 may be either rotated about the opticalaxis 56, as shown by the arrow 58, or different rotational orientationsof the reference character may be present on a slide and sequentiallyinserted into the optical system in order to achieve effective rotationof the displayed image. In order that the patient is not provided anyvisual clues as to the direction of rotation of the target 12, it isdesirable to provide a means for obscuring the target 12 duringrotation. However, it is also desirable that the screen 18 remainlighted during this rotation such that the patients eyes doe not have toreadjust to a different light level both before and after the. rotation.Such a function may be performed by inserting a diffuser 60 into theoptical system between the target 12 and the screen 18. Preferably thediffuser is inserted immediately in front of the target 12 and isslidably mounted as shown by the arrow 62. When the diffuser is inplace, the beam of light passing through the target 12 is sufficientlydiffused such that a constant background illumination is maintained onthe screen 18.

The total effective target size range may be integrally increased bysupplying a set of reference characters on the target 12 which arerelated in size by the ratio of magnification of the zoom lens system20. For example,-if the zoom lens system 20 were to supply amagnification range of 8: l a range of 64:1

may be achieved by providing two target sizes on a slide, the charactersizes being also in an 8:1 ratio.

At a 15-foot viewing distance, the smaller target size would provide aSnellen threshold from 20/15 to 20/120 and the larger character sizewould cover the range from 20/120 to 20/1 ,000. It similarly followsthat by working the patient at a -foot viewing distance, that theapparatus would afford a total acuity range of from 20/45 to 20/3,000.It is, however, quite likely that the majority of the acuitymeasurements would be accomplished while utilizing the larger sizetarget almost exclusively.

A means is included in the preferred embodiment of the invention, shownin FIG. 1, for directly reading the threshold level of the patient. Areference mark is placed on the target 12 adjacent the referencecharacter. This type of a mark is depicted by the mark 62 which is shownin FIG. 2. A set of graduated indicia 64 is emplaced on the screen 18.These may be a set of regularly spaced radial marks or a set ofconcentric circles depending on the type of target utilized. Therefore,when the patient acknowledges recognition of the image presented, theoperator is able to ascertain immediately quantitatively the acuity ofthe patient. This is accomplished by noting the coincidence of the mark62 on the graduated indicia 64, the distance the patient is positionedfrom the screen, and comparing a chart or nomogram. Alternatively, thisfunction could be performed by many other means, for example by sensingthe position of the cam follower, lead screw, zoom system barrel, or thecontrol switch and providing a readout therefrom.

The normal operating procedure for measuring the acuity by means of theaforementioned apparatus is as follows. The practitioner selects theappropriate viewing distance and reference character size based on hisexperience and knowledge of the patient. If the patients generalcondition is not known, the approach would be to start with thereference character presented at essentially the 20/15 position and thetest conducted therefrom. In other words, the practitioner presents theacuity target to the patient at below the patients threshold level. Thesize of the image is then gradually increased until the patient reportsthat he can see the image. In the above-described apparatus, the zoomoptics being electrically driven may be operated by the patient suchthat the practitioner is merely monitoring the test at this time. Thethreshold may then be varied by requiring the patient to identify themeridianal rotation of the image presented on the screen 18. Forrefinement of the measurement, the image size may be reduced prior toeach meridianal examination and the threshold measurement repeated.Obviously, in addition, the targets rotational flexibility may be alsoutilized where the correction of an astigmatic condition is appropriate.It is also foreseen, that for some patients the threshold measurementsmay need to be made at more than one viewing distance or at more thanone target size.

The above system provides greater convenience for both the clinician andthe patient and also affords far greater accuracy of measurement andrepeatability. This is accomplished by providing consistency of targetcontrast, precise control and calibration of the target size, and themeridianal flexibility allowable in the frontal plane.

While there has been shown and described what is considered to be apreferred embodiment of the present invention, it will be obvious tothose skilled in the art that various changes and modifications may bemade therein without departing from the invention.

lclaim:

1. Apparatus for measuring the acuity of a low-vision patient comprisingan effectively rotatable target having a character thereon,

a rear vision screen,

means for projecting a variably magnified image of the character ontothe rear vision screen,

means for rendering the image unintelligible to the patient while thecharacter is effectively rotated, means for controlling themagnification of the image whereby the acuity of the low-vision patientmay be measured by enlarging the image until the patient is able torecognize the character presented.

2. Apparatus according to claim 1, wherein means are provided forretaining illumination of the screen while the character is effectivelyrotated.

3. Apparatus according to claim 2, in which the means for rendering theimage unintelligible is a diffuser which is inserted between the targetand the screen.

4. Apparatus according to claim 1, wherein the means for projectingincludes a zoom lens system, the means for controlling compriseselectromechanical means connected to the zoom lens system for drivingthe zoom lens system between its limits of magnification and a switchwhich is operable by the patient for actuating the electromechanicalmeans, and the target includes a plurality of interchangeable charactersthereon, the characters being related in size by the ratio ofmagnification of the zoom lens system to integrally increase the rangeof continuous magnification.

5. Apparatus according to claim 1, wherein the target includes aradially disposed reference mark and the screen includes a set ofgraduated indicia for directly reading the threshold level ofa patient.

1. Apparatus for measuring the acuity of a low-vision patient comprisingan effectively rotatable target having a character thereon, a rearvision screen, means for projecting a variably magnified image of thecharacter onto the rear vision screen, means for rendering the imageunintelligible to the patient while the character is effectivelyrotated, means for controlling the magnification of the image wherebythe acuity of the low-vision patient may be measured by enlarging theimage until the patient is able to recognize the character presented. 2.Apparatus according to claim 1, wherein means are provided for retainingillumination of the screen while the character is effectively rotated.3. Apparatus according to claim 2, in which the means for rendering theimage unintelligible is a diffuser which is inserted between the targetand the screen.
 4. Apparatus according to claim 1, wherein the means forprojecting includes a zoom lens system, the means for controllingcomprises electromechanical means connected to the zoom lens system fordriving the zoom lens system between its limits of magnification and aswitch which is operable by the patient for actuating theelectromechanical means, and the target includes a plurality ofinterchangeable characters thereon, the characters being related in sizeby the ratio of magnification of the zoom lens system to integrallyincrease the range of continuous magnification.
 5. Apparatus accordingto claim 1, wherein the target includes a radially disposed referencemark and the screen includes a set of graduated indicia for directlyreading the threshold level of a patient.